The major objective of this project is to use a genetic approach to define the molecular organization and mechanochemical function of flagellar dynein arms. A series of flagellar motility mutants of Chlamydomonas reinhardtii have already been isolated and characterized with specific defects for the assembly and function of the different microtubule-associated dynein arms. Our strategy has been, and will continue to be, to identify arm-defective mutants on the basis of molecular deficiencies or alterations for polypeptide components of the structures (as derived from our studies on mutants deleted for specificdynein arms). These mutants will be characterized by methods already established for analysis of their genetic, ultrastructural, biochemical and flagellar motility phenotypes. In addition to these methods of analysis we propose to undertake two new approaches of analysis: (1) to apply low angle rotary shadowing techniques to resolve at the electron microscopic level high resolution details of the substructure of the dynein arms; and (2) to undertake in vitro reconstitution experiments to characterize arm-assembly defective mutants and to study the hierarchy of assembly of dynein complexes and non-catalytic subunits into arm structures. We anticipate that the characterization of mutants with specific lesions for the dynein arms (at the molecular to functional levels) are likely to contribute in a unique way to studies on the molecular requirements for the assembly and function of dynein arms in flagellar motility and other microtubule-based systems of movement.